In general, when a character written on a signboard, an indicator, a paper sheet, etc, is extracted from an image obtained by photography using a camera, an inconvenience as below will arise. Namely, a character in an image exhibits various deformed appearances (because of, for example, deformation due to projection), depending upon the positional relationship between a sheet surface printed with the character and the point of view of the camera. Therefore, it is difficult to reliably extract the character from the image.
In order to reliably extract a character from an image, a character detection method has been developed, in which a frame around the character is firstly detected, then an image where the detected frame is corrected to a non-deformed state is generated, and a non-deformed character is detected in the generated image.
In this character detection method, the character must be written within a known frame. If the character is written in a frame of an unknown shape, it cannot reliably be extracted from the image.